British light heavyweight Buatsi earned a win yet disappointing debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a fight where the approaches failed to mesh, with excessive holding marring the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They win," Buatsi said in the ringside as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the victory but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who utilized a tight guard to edge the more effective stanzas.
The spectators' acceptance decreased amid regular grabbing, with Parker falling to the floor various instances during grappling.
The underdog closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to win the European light-heavyweight championship.
The arena grew hushed for a deeply poignant homage before the fight as ten bells tolled in memory of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a roar of "there's just one Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was knocked down by a left hook squarely on the face in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged soon, significant bruising appearing under his eye as the duo landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and courageous, exactly what their hero the legend embodied.
